Two-color laser-induced fluorescent thermometry for microfluidic systems

The feasibility of implementing a two-color laser-induced fluorescence (LIF) technique to study thermal transport at the microscale is investigated. Temperature-sensitive (Rhodamine B) and temperature-insensitive (Sulforhodamine-101) fluorescent dyes are used in tandem to determine fluid temperature with high accuracy and low noise using a pulsed Nd:YAG laser as an illumination source. While the fluorescence intensity of the temperature-sensitive dye is proportional to temperature, it is also biased by variations in the illuminating intensity. Therefore, a second temperature-insensitive dye is required to compensate for such biases. Calibration of the two-color LIF system using the RhB-SR101 dye combination in ethanol and water yields temperature sensitivities of -1.5% K-1 and -2.7% K-1, respectively, with volumetric illumination from an Nd: YAG laser. The feasibility of this methodology for conducting temperature measurements is explored by measuring a steady-state temperature gradient maintained across a microfluidic channel array by two large hot and cold reservoirs. These measurements reveal that the mean steady-state temperatures in the microchannels are within +/- 0.4 degrees C and +/- 0.3 degrees C of the predicted temperatures with ethanol and water as the solvents, respectively, with a spatial resolution of 22.2 x 22.2 mu m. The experimental uncertainties in the measurements using the RhB-SR101 dye combination are +/- 0.48-0.59 degrees C and +/- 0.41-0.49 degrees C for ethanol and water, respectively.